Selective breeding for desirable traits is an established practice in animal husbandry. Form, function, and fashion have contributed to the popularity of certain sires of bloodlines in various breeds of horses.
As horses are selected for their athletic abilities, for instance, racing speed or gait, less desirable traits can be expanded in the equine gene pool. The genetic consequences of such intensive selective inbreeding of popular lines of horses have received attention recently. See, for example, Cunningham, P., Sci. Am. 92 (May, 1991), and Holden, C., Science 252:34 (1991), the disclosures of which are incorporated herein by reference. Such purposeful breeding of popular horses has resulted in a disproportionate representation of a small number of sires in the ancestry of all extant horses. In the example of Thoroughbreds, only four stallions are responsible for 30% of the current gene pool. Cunningham, P., Sci. Am. 92 (May, 1991). Diseases of horses with proven or probable genetic basis include HYPP, combined immunodeficiency disease of Arabian horses, cerebellar abiotrophy, osteochondrosis, and equine degenerative myeloencephalopathy, among others. See, Leipold, H. W., et al., Large Animal Internal Medicine (Ed. Smith, B. P.) 1567 (E. V. Mosby Co., St. Louis) (1990), the disclosure of which is incorporated herein by reference.
Quarter Horses are the most popular breed of horses in the United States and were originally bred in the 1600's to run the quarter mile. Pronounced muscularity is a desirable trait in Quarter Horses and is frequently mentioned in the American Quarter Horse Association description of the breed. Recently a sire possessing particularly well developed musculature contributed substantially to the genetic makeup of the 2.9 million Quarter Horses that are currently registered. Concurrently with the rise in popularity of this sire, HYPP, an autosomal dominant disease characterized by potassium-induced episodes of skeletal muscle paralysis has been frequently found in Quarter Horses. See, Spier, S. J., et al., J. Am. Vet. Med. Assoc. 197:1009 (1990), the disclosure of which is incorporated herein by reference. All affected horses have been traced back to this common ancestor.
HYPP in humans is also inherited as an autosomal dominant trait and ha s been extensively studied. The human disease is characterized by intermittent attacks of weaknesses or paralysis. Both HYPP and paramyotonia congenita, a clinically distinct dominant disorder typified by cold-induced involuntary muscle contraction (myotonia), have been shown to have been linked to the human adult skeletal muscle sodium channel gene on chromosome 17 g. See, Fontaine, B., et al, Science 250:1000 (1990); Koch, M. C., et al., Hum. Genet. 88:71 (1991); and Ptacek, L. H., et al., Am J. Hum. Genet. 49:851 (1991), the disclosures of which are incorporated herein by reference. Recently, mutations of the human sodium channel have been identified for both HYPP as reported by Rojas, C. V., et al., Nature 354:387 (1991) and Ptacek, L. J., et al., Cell 67:1021 (1991), the disclosures of which are incorporated herein by reference, and paramyotonia congenita as reported by Ptacek, L. J., et al., Neuron 8:891 (1992), and McClatchey, A. I., et al., Cell 68:769 (1992), the disclosures of which are incorporated herein by reference. All of these mutations cause single amino acid substitutions that affect sodium channel functions.
Because of the clinical, genetic, and electrophysiological similarities between the human disease and HYPP in Quarter Horses it has been suggested that they could represent homologous disorders. For example, both show fully penetrant dominant inheritance as described by Spier, S. J., et al., J. Am. Vet. Med. Assoc. (In press) 202:993 (1993), the disclosure of which is incorporated herein by reference, potassium-induced attacks as described by Ptacek, L. J., et al., Neuron 8:891 (1992), the disclosure of which is incorporated herein by reference, elevated serum potassium during attacks, and membrane potential abnormalities in isolated muscle cells as described by Pickar, J. G., et al., Am. J. Physiol. (Cell Physiol.) 260, C926 (1991), the disclosures of which are incorporated herein by reference.
In order to test this hypothesis a molecular genetic linkage study was conducted by Rudolf, J. A., et al., Animal Genetics 23:241 (1992), the disclosure of which is incorporated herein by reference, in part of a large HYPP Quarter Horse pedigree using a polymerase chain reaction (PCR)-based polymorphism that was discovered while horse sodium channel .alpha.-subunit cDNA was being sequenced. The study revealed no recombinants between this particular polymorphism and the trait in ten informative meioses (LOD Z=2.7; .theta.=0). The analysis indicated that the sodium channel alpha subunit was a likely candidate gene for HYPP in Quarter Horses.
Horses affected with HYPP exhibit episodes of weakness and collapse which pose a hazard to themselves and their riders. Since affected horses usually do not exhibit signs of the disease until they are mature and in training, breeders and sellers of young horses often do not know if offspring of affected horses carry the trait. At the present time known methods of testing for HYPP are inadequate, such as, potassium chloride challenge which can lack sensitivity and spontaneous activity by electromyography which is not specific for HYPP. Therefore there is a need for a realiable and accurate diagnostic test to diagnose this disease and help eliminate this disease from horses.
The present invention provides the identification of the mutation that causes HYPP in horses and methods of diagnosing the HYPP disease by detecting or identifying the mutation.